In recent years, image compression technologies have become essential in cases where image data are transmitted on a narrowband channel or stored in a storage medium of small capacity. MPEG (Moving Picture Experts Group) as a media coding standard of the International Standardization Organization (ISO) is a typical one of these image compression technologies, and has come into wide use. In a system compliant with MPEG-4 which is the newest in MPEG standards and is being put to practical use, it is possible to carry out decoding processes for plural media streams in parallel and compose images corresponding to the respective streams to display the same.
FIG. 8 is a block diagram for explaining a prior art decoder. In this case, a media stream is one which is obtained by coding digital data corresponding to images, audio or the like composing one scene.
This prior art decoder 800 comprises a demultiplexing means 802 for carrying out a demultiplexing process for a multiplexed stream 801 which is obtained by multiplexing plural streams (in this prior art, two streams are taken as an example) on the side of an encoder, to separate the multiplexed stream into plural streams (in this case, two streams 803 and 806), a first decoding means 804 for decoding the stream 803 and outputting a decoded image data 805, a second decoding means 807 for decoding the stream 806 and outputting a decoded image data 808, and a composition means 809 for composing the decoded image data 805 and 808 and outputting a composed image data 810.
Next, the operation of the decoder 800 will be described.
In this decoder 800, the demultiplexing means 802 separates the input multiplexed stream 801 into the streams 803 and 806 by the demultiplexing process. Then, the first decoding means 804 decodes the stream 803 and outputs the decoded image data 805. Similarly, the second decoding means 807 decodes the stream 806 and outputs the decoded image data 808. When the decoded image data 805 and 808 are input to the composition means 809, the composition means 809 composes the decoded image data 805 and 808, and outputs the composed image data 810 to a display monitor (not shown). Thereby, on the display monitor, a composed image is displayed on the basis of the composed image data 810.
In the so-constructed decoder, the decoding processes for the respective streams are carried out in parallel by the decoding means corresponding to the respective streams, thereby composing images or the like corresponding to the respective streams to display the same.
Thus, the above-mentioned prior art decoder decodes the plural streams included in the multiplexed stream by the decoding means corresponding to the respective streams, and thus requires as many of the decoding means as the streams. Thus, the costs are high, especially for portable devices having such a decoder.
To be more specific, the above-mentioned decoding means can normally be realized by software or special hardware, while in portable devices decoding means realized by special hardware are often mounted to reduce power consumption. However, when a plurality of special hardware such as the decoding means are used in electronic devices such as the decoders, the costs of the devices are increased and, in some cases, the devices are upsized.